1. Field of the Invention
This invention relates to a resin composition for forming a porous film having a good water vapor permeability and air permeability and a superior water resistance and a porous film prepared therefrom.
2. Description of the Related Art
Porous films have so far been obtained by filling various non-compatible substances in polyolefin resins, making the resulting compositions into films and stretching the films (see Japanese patent publication No. Sho 53-12542/1978, U.S. Pat. No. 4,347,844 and Japanese patent application laid-open No. Sho. 57-59727/1982).
However, such films obtained according to these processes have such drawbacks that they are weak in the mechanical strengths, no uniform pore distribution is formed and they are too rigid, resulting in a less-flexible film. Thus, for example, as proposed in G.B. No. 2,151,538, the above problems have been improved by using barium sulfate as the non-compatible substance, but such use of barium sulfate is liable to cause clogging of the screen of extruder due to its flocking and also to lower the screen life and further to cause stretching breakage due to its flocks; hence such a process has not yet been satisfactory in the aspect of stabilized productivity. The screen life referred to herein will be defined in Examples.